For noise reduction circuits for reducing noise in digital signals, so-called recursive noise reduction circuits are known in which a memory is used. Especially, these recursive noise reduction circuits are widely used when performing noise reduction on digital video signals.
General conventional recursive noise reduction circuits process the interlace video signals each configured by even-numbered field images and odd-numbered field images typified by NTSC and PAL schemes.
For example, the analog VCR (Video Cassette Recorder) is capable of not only providing normal speed playback but also providing irregular playback operations such as fast-forward, rewind, and picture search.
In the case of interlace video signals, a signal which is reproduced at the normal speed has a shift by 0.5 line between the horizontal sync pulses of the video signals of the even-numbered field and the odd-numbered field with reference to the vertical sync pulse. Such a signal is hereafter referred to as a standard signal.
On the other hand, in a video signal which is reproduced by an irregular playback operation such as fast-forward, the horizontal sync pulses of the video signals of the even-numbered field and the odd-number field are in phase with reference to the vertical sync pulse. Such a signal is hereafter referred to as a nonstandard signal. It should be noted that the nonstandard signal sometimes includes those standard signals which are low in S/N and video signals which are outputted from game machines.
Now, it is assumed that a general recursive noise reduction circuit be installed on a television receiver or the like in which not only the standard signal but also the nonstandard signal outputted from an analog VCR or the like mentioned above is inputted.
In this case, the noise component of each inputted standard signal is normally reduced by the installed general recursive noise reduction circuit.
However, because the horizontal sync pulses of the video signals of the even-numbered field and the odd-numbered field are in phase in the inputted nonstandard signal, the recursive noise reduction circuit cannot correctly determine whether the current field is the even-numbered field or the odd-numbered field. And, if a noise reducing operation including the interpolation processing by an interpolation filter 2 and an interpolation filter 5 is executed in this state, the effective reduction of noise cannot sometimes be achieved.
As one of examples of the above-mentioned inappropriate noise reducing operation, the noise to be reduced looks moving upward or downward on the display screen. When the noise to be reduced by executing a noise reducing operation moves, the noise becomes more conspicuous than the case in which no noise reducing operation is executed, leaving noise unmoved, thereby making the video image visually undesirable.
To solve the above-mentioned problem, a circuit for determining whether an inputted signal is the standard signal or the nonstandard signal may be added to a general recursive noise reduction apparatus to execute a noise reducing operation only when the inputted signal is the standard signal.
However, such a circuit having a simple circuit configuration and capable of determining with stability whether an inputted signal is the standard signal or the nonstandard signal has not been available.